Reply to the Comment "Past of a quantum particle and weak measurement"

Misinterpretation in the preceding Comment of my recent analysis of the past of a photon is corrected. There is nothing in this analysis which is "contrary to the usual quantum expectations" but, nevertheless, it does provide "further understanding and interpretation of the system considered". In particular, it teaches us that the naive common sense argument frequently used in the literature including the Comment has to be abandoned.Comment: 2 pages, to be published in PRA, Reply to arXiv:1306.304

Weak value controversy

Recent controversy regarding the meaning and usefulness of weak values is reviewed. It is argued that in spite of recent statistical arguments by Ferrie and Combes, experiments with anomalous weak values provide a useful amplification techniques for precision measurements of small effects in many realistic situations. The statistical nature of weak vales was questioned. Although measuring weak value requires an ensemble, it is argued that the weak value, similarly to an eigenvalue, is a property of a single pre- and post-selected quantum system

Do neutrons disagree with photons about where they have been inside an interferometer?

Recent experiments with identically tuned nested Mach-Zehnder interferometers which attempted to observe the location of particles inside these interferometers are analyzed. In spite of claims to the contrary, it is argued that all experiments support the same surprising picture according to which the location of the particles inside the interferometers is not described by continuous trajectories.Comment: To appear in PR

The analysis of Hardy's experiment revisited

Cohen and Hiley [Phys. Rev. A 52, 76 (1995)] have criticized the analysis of Hardy's gedanken experiment according to which the contradiction with quantum theory in Hardy's experiment arises due the failure of the "product rule" for the elements of reality of pre- and post-selected systems. It is argued that the criticism of Cohen and Hiley is not sound.Comment: 3 pages, LaTe

Comment on "How the result of a single coin toss can turn out to be 100 heads"

In a recent Letter [PRL 113, 120404 (2014)] Ferrie and Combes claimed to show "that weak values are not inherently quantum, but rather a purely statistical feature of pre- and post-selection with disturbance." In this Comment I will show that this claim is not valid. It follows from Ferrie and Combes misunderstanding of the concept of weak value.Comment: Comment on arXiv:1403.236

Defending Time-Symmetrized Quantum Theory

Recently, several authors have criticized time-symmetrized quantum theory originated by the work of Aharonov et al. (1964). The core of this criticism was the proof, which appeared in various forms, showing that counterfactual interpretation of time-symmetrized quantum theory cannot be reconciled with the standard quantum theory. I argue here that the apparent contradiction appears due to inappropriate usage of traditional time asymmetric approach to counterfactuals, and that the contradiction disappears when the problem is analyzed in terms of time-symmetric counterfactuals. I analyze various aspects of time-symmetry of quantum theory and defend the time-symmetrized formalism.Comment: 19 pages, LaTe

Weak Measurements

Weak measurement is a standard measuring procedure with two changes: it is performed on pre- and post-selected quantum systems and the coupling to the measuring device is weakened. The outcomes of weak measurements, ``weak values'' are very different for the eigenvalues of the measured operators. The weak values yield novel rich structure of the quantum world. Weak values help explaining peculiar quantum phenomena and finding new effects which might have practical applications.Comment: 17 pages, 4 figures, TAUP 2194-94, for Proc. of Erice School 94

The Two-State Vector Formalism

The two-state vector formalism (TSVF), the time-symmetric description of the standard quantum mechanics originated by Aharonov, Bergmann and Lebowitz is reviewed. The TSVF describes a quantum system at a particular time by two quantum states: the usual one, evolving forward in time, defined by the results of a complete measurement at the earlier time, and by the quantum state evolving backward in time, defined by the results of a complete measurement at a later time.Comment: 6 pages, entry for a forthcoming "Compendium of Quantum Physics

Comment on "One-state vector formalism for the evolution of a quantum state through nested Mach-Zehnder interferometers"

Bartkiewicz et al. [Phys. Rev. A 91, 012103 (2015)] provided an alternative analysis of experiment performed by Danan et al. [Phys. Rev. Lett. 111, 240402 (2013)] which presented surprising evidence regarding the past of photons passing through an interferometer. They argued that the quantity used by Danan et al. is not a suitable which-path witness, and proposed an alternative. It is argued that the quantum and classical analyses of Bartkiewicz et al. are inconsistent and both are inappropriate for describing the past of photons in a properly working interferometer.Comment: Comment on arXiv:1410.845

Weak Value and Weak Measurements

The weak value of a variable O is a description of an effective interaction with that variable in the limit of weak coupling. It is particularly important for a pre- and post-selected quantum system.Comment: 4 pages, entry for a forthcoming "Compendium of Quantum Physics